If
erected on the bank of an east and west stream, the north end of
the ship will become the north pole of a magnet and the south end
the south pole. Accordingly, when she is launched and proceeds to
sea, the compass points not exactly according to the magnetism of
the earth, but partly according to that of the ship also.
The methods of obviating this difficulty have exercised the
ingenuity of the ablest physicists from the beginning of iron ship
building. One method is to place in the neighborhood of the
compass, but not too near it, a steel bar magnetized in the
opposite direction from that of the ship, so that the action of
the latter shall be neutralized. But a perfect neutralization
cannot be thus effected. It is all the more difficult to effect it
because the magnetism of a ship is liable to change.
The practical method therefore adopted is called "swinging the
ship," an operation which passengers on ocean liners may have
frequently noticed when approaching land. The ship is swung around
so that her bow shall point in various directions. At each
pointing the direction of the ship is noticed by sighting on the
sun, and also the direction of the compass itself. In this way the
error of the pointing of the compass as the ship swings around is
found for every direction in which she may be sailing.
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